Biophotonics_Concepts_to_Applications

7.5.1 Mach-Zehnder Interferometer.

Owing to their versatile setup configurations, Mach-Zehnder interferometers are
popular for diverse biosensing applications, for example, reflectometric interference
spectroscopy. In a basicMach-Zehnder interferometer(MZI) light from a laser
source is divided into two paths by means of a beamsplitter or a 3-dB optical coupler,
as shown in Fig.7.19. These paths are called thearmsof the interferometer. One
path of the MZI is asensing armand the other path is areference arm. In the sensing
arm an interaction between light and a biosensor, for example, a receptor-analyte
complex as described in Sect.7.4, takes place by means of evanescentfield waves.
The reference arm is kept isolated from the external parameter being analyzed. After
the light passes through the two arms, a beam combiner or a 3-dB optical coupler
recombines the two waves. The constructive and destructive interference pattern
resulting from the recombination of the two lightwaves then is directed to an optical
signal analyzer, which makes an assessment of the parameter being evaluated. For
example, the optical signal variations can directly measure the concentration of the
analyte interacting with the sensing arm.
In order to create a more compact and robust interferometer design, biosensors
based on an all-fiber inline MZI scheme have been proposed and implemented. The
designs include a taperedfiber configuration [ 33 , 34 ], a microcavity imbedded in a
fiber [ 35 ], the use offiber Bragg gratings [ 36 ], cascaded segments of differentfibers
[ 37 – 39 ], and photonic crystalfibers that are selectivelyfilled with a liquid.
As one example using an FBG, Fig.7.20shows a sensor structure with two
long-period gratings embedded in afiber core. Along-period grating(LPG) is a
periodic structure in afiber core that couples co-propagating modes in opticalfibers
[ 40 – 42 ]. The coupling is realized through phase matching between the fundamental
LP 01 guided core mode and higher-order cladding modes. When light traveling in the
fiber core encounters thefirst grating, part of the light is coupled into thefiber cladding
and the other part continues propagating in thefiber core. The light in the cladding then
interacts with an external parameter through evanescent wave coupling.
Subsequently,the perturbed claddingwaveform reenters thefiber core bymeans of the
second grating and is combined with the light traveling in the core reference path. The

Input

optical

signal

3-dB optical

coupler or

beamsplitter

3-dB optical

coupler or

beam combiner

Reference arm

Sensing arm

Light/analyte interaction

To optical

signal

analyzer

Fig. 7.19 Operation of a basic Mach-Zehnder interferometer

7.5 Interferometric Sensors 217